Hide "[WARNING] Could not deduce format from file extension .pdc, default to markdown"

[anntzer]

When compiling a faulty file, e.g.

\oops

with :Pandoc pdf, if the file has e.g. a .pdc extension (one of the default extensions recognized by vim-pandoc as "pandoc's markdown"), the error window prints

# Press q to close this
> pandoc  --to="latex" --output="foo.pdf" --pdf-engine="lualatex" "foo.pdc"
[WARNING] Could not deduce format from file extension .pdc
  Defaulting to markdown
Error producing PDF.
! Undefined control sequence.
l.55 \oops

The first two lines regarding the file format are uninformative and very slightly distracting; they could be removed by passing --from markdown to the pandoc process (after all, vim-pandoc certainly knows that it is passing in a markdown file).